Wireless telecommunications systems, such as the well-known cellular and cordless telephone and data transmission systems, typically provide duplex radio communication between fixed radio access units, also called radio base stations or radio fixed parts, and remote portable radio communication units, also called portable parts. Each fixed radio access unit provides a number of radio communication channels in a geographically limited area, called a cell. Typical coverage areas are picocells (range of a few meters), nanocells (range up to 50 m) and microcells (range 10 to 400 m).
Typically, each radio access unit provides a number of radio communication channels available to the users in a cell. While moving across the coverage or service area of a radio telecommunication system, once the remote portable radio communication unit leaves the coverage area of a radio access unit, a call in progress is handed over from one radio access unit to the other dependent on a so-called handover threshold.
Channel selection and channel occupation during handover is a proces which involves a considerable amount of signalling load, whereas the speech or data transmission quality can be adversely affected if no proper channel can be selected and occupied. In the latter case, a call in progress can be severely distorted by a number of mutes or even be dropped.
International patent application WO 96/06512 discloses an improved handover procedure in a cellular mobile radio communication system, comprising a number of base stations organized into a cell structure wherein the cells are arranged in at least two layers in a hierarchical manner. There is disclosed a systematic way of passing between layers by a roaming portable radio communication unit, providing suitable handovers upwards and downwards through the cell hierarchy.
In the home or residential domain, radio communication is typically performed between a single fixed radio access unit or home base station and a few portable radio communication units, such as portable radio telephones and/or portable data equipment. The radio access unit connects to the Public Switched Telephone Network (PSTN) or an Integrated Services Digital Network (ISDN), for example.
Business radio communications systems, such as Business Cordless Telecommunications (BCT) systems, are capable of serving hundreds or even thousands of portable radio communication units by a plurality of fixed radio access units spread across (large) offices, production halls, etc. The radio access units operatively connect to a Central Interface Unit (CIU) or Radio Exchange (RE) which, in turn, connects to a Private Automatic Branch eXchange (PABX), the PSTN or the ISDN, for example.
Outdoor communications services, such as Cordless Terminal Mobility (CTM), for example, cover a whole town or city. Microcell coverage is provided by a large amount of fixed radio access units positioned across the town or city. Next to, or instead of, fixed radio access units serving such relatively small cells, fixed radio access units providing service to macrocells having a range of 1 to 5 km are applied with CTM.
Radio communications systems typically operate according to a radio communications standard, such as the pan European Global System for Mobile telecommunications (GSM), the Digital Enhanced Cordless Telecommunications (DECT) and other standards, designated AMPS, D-AMPS, ETACS, NMT, PHS, etc. Principal transmission techniques used are TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access) and CDMA (Code Division Multiple Access).
In particular with the availability of outdoor services, such as CTM, users will desire to use one and the same portable telecommunication unit (e.g. a portable telephone) at home, in the office and/or on the street.
An ever lasting concern with portable radio communication equipment is, however, its scarce battery power. It will be appreciated that, for being able to use one and the same portable communication device throughout the several radio environments, attention has to be paid to a reduced battery consumption, in order to maximize stand-by and operation times of the device.